1. Field of the Invention
The present invention relates to a composite metallic material having a metallic material substrate and a polymer thin film formed on the surface of the substrate, a method for manufacturing the composite metallic material, an etched metallic material obtained by forming etching pits in the composite metallic material, a method for manufacturing the etched metallic material, and an electrolytic capacitor.
In this specification, the wording of “aluminum” is used to mean both aluminum and its alloy.
2. Background Art
In recent years, in view of the demands of miniaturization of electronic devices, high density mounting of printed circuit boards and effective mounting thereof, it has been remarkably developed to manufacture chip-shaped miniaturized electronic parts. In view of this, demands of increasing capacitance of electrolytic capacitor have been increased.
Generally, an aluminum foil to be used for electrolytic capacitors is subjected to etching processing to increase the electric capacitance by increasing the surface expansion ratio. Furthermore, since the surface expansion ratio increases as the number and length of the etching pit to be formed by the etching processing increases, various processing is performed to an aluminum material. For example, such processing includes controlling to (100) crystal orientation, adjusting the composition by adding trace elements such as Cu or Pb into an aluminum material, degreasing washing before final annealing and formation processing of crystalline oxide film during final annealing (see, Japanese Examined Laid-open Patent Publication No. 58-34925, Japanese Unexamined Laid-open Patent Publication No. 3-122260, etc.).
However, simply increasing the number or the length of etching pits has been approaching to a limit for increasing the capacitance. In order to increase the surface expansion ratio of an aluminum foil, it is necessary to generate etching pits uniformly with high density on the etching surface by decreasing partial etching, etching failure and surface melting. However, in the aforementioned methods, it is not enough to generate etching pits uniformly with high-density, and the method do not meet the demand of capacitance which are being further increased.
Accordingly, in currently available aluminum foils, distribution of pitting pits can not be controlled during etching processing, causing joined tunnel-like pits, which in turn loses the surface area to be enlarged. Thus, the current capacitance is merely 50 to 65% to ideal capacitance. For the purpose of increasing capacitance, although various attempts such as adhering foreign substances or mechanically forming defects have been made, no attempt has become successful.